The Mystery League, Inc. was an early 1930s publisher of mysteries/thrillers. Only 30 were published. The series was distinctive for two reasons - the books were sold in a drug store and a cigar store chain for 50 cents, less than the going hardcover price and the dust jackets, which is the main reason they are collectible.
The Mystery League series shares three authors with Harlequin Books. Here's the first - Frances Shelley Wees with one Mystery League and two Harlequins.